U.S. Pat No. 4,944,854 to Felton et al. states that it is known that certain dielectric materials can be permanently electrostatically polarized and that these materials are polarized by, (1) exciting the material by heating, (2) applying a high-voltage electric field, and (3) cooling the material while under the influence of the electric field. Upon removal of the electric field, appropriate dielectric materials will be found to have become the electrical analog of a permanent magnet. A dielectric becomes an electret when the rate of decay of the field-induced polarization can be slowed down so much that a significant fraction of the polarization is preserved long after the polarizing field has been removed.
The Felton et al. '854 patent continues by stating that early methods for the formation of fibrous electrets from thermoplastic films or filaments involved disposing the thread or filaments in an electrostatic field established between parallel closely spaced electrodes. This process, which is disclosed in U.S. Pat. No. 2,740,184, heats the thread or filamentary material to soften it and then cools the material in the presence of the electrostatic field whereupon charges are introduced into the fibers. The voltage employed in charging the material is 4000 volts or more of direct current. The filamentary material itself is a hollow filamentary material having a wax core such as carnauba wax, the resultant product being described as having a "more or less permanent charge". In order to produce that degree of permanence of charge in a non-wax electret, such as for instance a polyolefin electret, it has been found that substantially higher voltages must be employed, that is to say direct current voltages in the range of 8000 volts or more. When such higher voltages are employed, the electret field will break down and arcing will occur in the free air space surrounding the single fiber or filamentary material employed according to the teachings of U.S. Pat. No. 2,740,184.
The Felton et al. '854 patent further states that arcing produced from high voltages, that is to say voltages of 8000 volts direct current or higher, may be circumvented by covering the electrodes with a poorly conductive material so as to evenly distribute the applied voltage and dampen possible dielectric breakdown. For instance, U.S. Pat. No. 3,571,679 discloses a process for forming electrets by closely winding a monofilament fiber such as a polypropylene fiber on a hollow winding roller which has been previously surfaced with a polyamide faced aluminum foil. Subsequent to winding the layer of fibers, a second polyamide faced aluminum foil is wound about the yarn layer. The fiber and foil wound roll is then disposed between two electrodes where it is polarized for three hours at a temperature of about 120.degree. C. with a voltage of 200 volts. This method, however, is discontinuous and extremely slow in that charging times of about three hours for the wrapped roll are required.
The Felton et al. '854 patent also states that as a result of such deficiencies, electrets were then commonly produced by either a spray spinning technique such as that set forth in U.S. Pat. No. 4,215,682 wherein an electric charge is introduced into meltblown fibers during the melt-blowing process, or alternatively, the electrets are prepared from a film which is homopolarly charged and subsequently fibrillated (see U.S. Pat. No. 3,998,916).
Other background materials dealing with electrets include (1) the Introduction of Topics in Applied Physics, G. M. Sessler, Vol. 33, 2nd ed., 1987, pp 1-12.; (2) Recent Progress in Electret Research, Topics in Applied Physics, R. Gerhard-Multhaupt et al., Vol. 33, 2nd ed., 1987, pp 383-431 and (3) Electrets and Related Electrostatic Charge Storage Phenomena, L. M. Baxt et. al., The Electrochemical Society, Inc. 1968 (LC no. 68-23768).
One of the difficulties with which those in the art have been faced with is the ability to form an electret which may be used in filtration devices and which is formed from a thin film material without having to subsequently fibrillate the film into fibers or filaments.
As can be seen in the paragraph citing related applications, we have filed several patent applications dealing with applications of hydrosonic energy. A starting point in the understanding of the principles of hydrosonic energy is a fundamental knowledge of ultrasonics. Ultrasonics is basically the science of the effects of sound vibrations beyond the limit of audible frequencies. Ultrasonics has been used in a wide variety of applications. For example, ultrasonics has been used for (1) dust, smoke and mist precipitation; (2) preparation of colloidal dispersions; (3) cleaning of metal parts and fabrics; (4) friction welding; (5) the formation of catalysts; (6) the degassing and solidification of molten metals; (7) the extraction of flavor oils in brewing; (8) electroplating; (9) drilling hard materials; (10) fluxless soldering and (10) nondestructive testing such as in diagnostic medicine.
The object of high power ultrasonic applications is to bring about some permanent physical change in the material treated. This process requires the flow of vibratory power per unit of area or volume. Depending on the application, the power density may range from less than a watt to thousands of watts per square centimeter. Although the original ultrasonic power devices operated at radio frequencies, today most operate at 20-69 kHz.
The piezoelectric sandwich-type transducer driven by an electronic power supply has emerged as the most common source of ultrasonic power; the overall efficiency of such equipment (net acoustic power per electric-line power) is typically greater than 70%. The maximum power from a conventional transducer is inversely proportional to the square of the frequency. Some applications, such as cleaning, may have many transducers working into a common load.
Other, more particular areas where ultrasonic vibratory force has been utilized are in the areas of thin nonwoven webs and thin films. For example, ultrasonic force has been use to bond or weld nonwoven webs. See, for example, U.S. Pat. Nos. 3,575,752 to Carpenter, 3,660,186 to Sager et al., 3,966,519 to Mitchell et al. and 4,695,454 to Sayovitz et al. which disclose the use of ultrasonics to bond or weld nonwoven webs. U.S. Pat. No. 3,488,240 to Roberts, describes the use of ultrasonics to bond or weld thin films such as oriented polyesters.
Ultrasonic force has also been utilized to aperture nonwoven webs. See, for example, U.S. Pat. Nos. 3,949,127 to Ostermeier et al. and 3,966,519 to Mitchell et al.
Lastly, ultrasonic force has been used to aperture thin film material. See, for example, U.S. Pat. No. 3,756,880 to Graczyk.
Other methods for the aperturing of thin film have been developed. For example, U.S. Pat. No. 4,815,714 to Douglas discusses the aperturing of a thin film by first abrading the film, which is in filled and unoriented form, and then subjecting the film to corona discharge treatment.
As has previously been stated, those in the art had recognized that ultrasonics could be utilized to form apertures in nonwoven webs. See, U.S. patent to Mitchell, et al. Additionally, the Mitchell et al. patent discloses that the amount of ultrasonic energy being subjected to a nonwoven web could be controlled by applying enough of a fluid to the area at which the ultrasonic energy was being applied to the nonwoven web so that the fluid was present in uncombined form. Importantly, the Mitchell, et al. patent states that the fluid is moved by the action of the ultrasonic force within the nonwoven web to cause aperture formation in the web by fiber rearrangement and entanglement. The Mitchell et al. patent also states that, in its broadest aspects, since these effects are obtained primarily through physical movement of fibers, the method of their invention may be utilized to bond or increase the strength of a wide variety of fibrous webs.